In nanotechnology, metal particles are typically used as catalysts for growth of nanowires and nanotubes (hereinafter nanomaterials). Each metal particle nucleates a single nanomaterial. As a result, the base location of each nanomaterial corresponds to the location held by the particulate catalyst from which it is nucleated. Accurate positioning of the metal catalyst provides a way to control the location of the nanomaterials. The diameter of the metal catalyst particle also defines the diameter of the nanomaterial. To obtain a tight distribution of nanomaterial diameters, metal colloids are typically used as the source of the metal particles. As is known to those skilled in the art, metal colloids have a tight size distribution of suspended particles made to a specified dimension. The fine particles are suspended in a liquid, and are typically charged to prevent them from forming clusters. Therefore, a method that provides an easy and robust way to deposit catalyst particles from a suspension onto selected regions of a wafer is desirable as a way to control both the nanomaterial location and diameter.
Conventional methods for obtaining metal particles at selected regions include, for example, blanket deposition of the particles followed by masking of a selected region and removal of the unmasked particles (for example, by etching). An example of such a method is illustrated in prior art FIGS. 1A-1E. The starting substrate 101 consists of a silicon substrate on which a thermal silicon dioxide (SiO2) layer 102 is formed (See, FIG. 1A). Particles 103 are dispensed on the SiO2 layer 102 providing the structure shown, for example, in FIG. 1B. The particles 103 can be dispensed by methods such as spraying, or by spinning a suspension. A photoresist film 104 is spun over the wafer and patterned by lithography to mask those regions where particles 103 are to remain providing the structure shown in FIG. 1C. Unprotected particles 103, as shown in FIG. 1D, are then removed by etching. Finally the photoresist 104 is removed by a solvent or by exposure to oxygen plasma. The resultant structure including the particles 103 on selected regions of the structure is shown in FIG. 1E.
The conventional methods, as illustrated by FIGS. 1A-1E, introduce some issues. First, for growth of nanowires it is imperative that the particles will be deposited directly on a clean silicon surface. The removal of the silicon native oxide is typically carried out by etching in diluted hydrofluoric acid (HF) that hydrogen terminates the surface and renders it hydrophobic (repelling water). Many suspensions consist of an aqueous solution in which the particles are suspended. Due to the hydrophobic nature of the silicon surface, the suspension will not wet the surface and, as a result, the particles will wash away. That is why in the example shown in FIGS. 1A-1E the substrate surface consists of a SiO2 film 102 that is hydrophilic and thus it is easily wetted by the suspension. Second, the use of a mask (for protecting the particles) will, in many instances, introduce additional issues. For example and in the case of a photoresist mask, the stripping of the resist with a solvent can wash or relocate the particles, while stripping by oxygen plasma will oxidize the silicon surface. Additionally, the use of a photoresist can introduce contamination by organic products.
Given the above challenges with the prior art a method that will allow the deposition of fine particles from a suspension directly onto designated regions of a clean hydrogen terminated silicon surface is desirable. The term “fine particles” is used throughout the instant application to denote particles having a typical size of about 1 to about 100 nm.